1. Field of the Invention
The present invention relates to distributed medium access scheduling with implicit ordering (hereinafter, referred to as “DSIO”), and more particularly, to DSIO which is capable of realizing higher throughput than a distributed coordination function (DCF) at all times by substituting for the DCF as a medium access control method which is basically used in the IEEE 802.11 wireless LAN.
2. Description of the Related Art
Recently, the IEEE 802.11 wireless LAN has been widely used for wireless Internet access, because of the simplicity of installation and a low cost. Currently, the IEEE 802.11 wireless LAN defines two different media access control schemes.
A basic access method uses the DCF which may be used for both an ad-hoc network environment and an infra-structured network environment, as a general carrier sense multiple access with collision avoidance (CSMA/CA) scheme.
Furthermore, a medium access control scheme called a polling-based point coordination function (PCF) may be selectively used.
When a wireless communication system has broadcasting characteristics and uses channels of the same band, two terminals cannot transmit signals at the same time. When two or more terminals transmit signals at the same time, a collision may occur, and thus all transmissions are failed.
FIG. 1 shows an internal structure of a transmission unit when a conventional DCF is used.
Referring to FIG. 1, when the conventional DCF is used, the transmission unit includes three inter frame spaces (IFS), that is, a short inter frame space (SIFS), a PCF inter frame space (PIFS), a DCF inter frame space (DIFS), in order to implement the IEEE 802.11 media access control scheme.
The IFS refers to a minimum waiting time which is necessarily required before a next frame transmission operation is performed after one frame is transmitted.
The relation among the respective IFSs is set as follows: SIFS<PIFS<DIFS. Since the SIFS is the shortest, the SIFS is used for a communication having the highest priority.
Hereinafter, a general method in which a collision occurring in a wireless environment is stochastically avoided through the DCF will be described.
Before all terminals transmit data, the DCF selects an arbitrary back-off number as a lower value than a contention window value, in order to avoid a collision between the transmission terminals.
The initial contention window value is set to CWmin, and it is checked whether a transmission medium is occupied by another terminal or not. At this time, when the transmission medium is not used during a predetermined DIFS, this state is considered as an idle state, and back-off is started.
When a transmission of another terminal is not performed during a predetermined slot time, the back-off number is decreased by ones. When the back-off number becomes zero, data is transmitted. When the transmission of another terminal is recognized, the back-off is stopped, and when the transmission medium becomes idle during the DIFS, the back-off is resumed.
When the back-off number becomes zero and the transmission medium is idle, transmission is started. A terminal receiving the data transmits an ACK after a predetermined SIFS to acknowledge a successful transmission.
The SIFS is smaller than the DIFS, and the transmission of another terminal does not interfere with the transmission of the ACK frame through the SIFS. When an ACK frame is not received, it is considered that the transmission is failed, and the contention window is doubled.
The maximum size of the contention window is CWmax. When a transmission is successfully performed, the contention window is reset to CWmin.
The conventional DCF has been essentially adopted as the medium access control scheme of the IEEE 802.11 wireless LAN. However, the conventional DCF generally exhibits low performance. In particular, as the number of terminals increases, collisions frequently occur. Back-off slots which are not used may cause the waste of bandwidth. As a result, the conventional DCF has a low transmission rate.
Meanwhile, a variety of schemes have been proposed to solve the above-described problems. However, in a general contention-based distributed medium access control scheme, a trade-off relation is established between collisions and back-off slots which are not used. In this case, when one is to be reduced, another is increased. Therefore, there is a limit in realizing desired performance improvement through such an access control scheme.